Processes which comprise making an image data visible from an electrostatic image such as electrophotographic process are used in various fields. In electrophotographic process for example, an electrostatic image is formed on a photoreceptor at the charging and exposure step. The electrostatic latent image is then developed with a developer containing a toner. The toner image thus developed is transferred, and then fixed to give a visible image. The developers to be used in this process can be classified as binary developer consisting of a toner and a carrier and unitary developer comprising a magnetic toner or nonmagnetic toner alone. Such a toner is normally produced by a knead-grinding process which comprises melt-kneading a thermoplastic resin with a pigment, an electrostatic controller and a releaser such as wax, cooling the mixture, and finely grindirng the mixture, and then classifying the particles. If necessary, the particulate toner thus obtained may occasionally comprise a particulate inorganic material or particulate organic material attached to the surface thereof to have improved fluidity or cleaning properties.
On the other hand, as the society has been oriented towards information more and more, there has recently been a growing demand for provision of data documents prepared by various methods in the form of image having a higher quality. To this end, studies have been made of enhancement of image quality in various image formation methods. This demand has been given to all image formation methods, not excepting one using electrophotographic process. In electrophotographic process, it has been desired to reduce the particle diameter of toner particles and attain a sharp particle size distribution in order to realize an image having a higher precision in the formation of color image.
In the operation of digital full-color copying machines or printers for example, the color of a color image original is subjected to separation through various filters (B (blue), R (red), G (green)). Latent images composed of dots having a diameter of from 20 to 70 .mu.m corresponding to the original are then subjected to development with the respective developer (Y (yellow), M (magenta), C (cyan), Bk (black)) by subtractive mixing action. This process requires that a larger amount of developers be transferred than by the conventional black-and-white copying machines. This process further requires that the development be effected corresponding to dots having a smaller diameter. Thus, it becomes more important to secure uniform chargeability including environmental dependence of charging, continuance of uniform chargeability, sharp particle size distribution and sufficient toner strength. Further, taking into account the growing demand for increase in the operation speed of these machines and energy saving, it has been desired to further lower the lowest temperature at which the toner image can be fixed. As obvious also from this fact, a toner having a small particle diameter with a sharpparticle size distribution has been desired.
However, in accordance with the grinding and classification process by the conventional knead-grinding method, the minimum particle diameter which can be actually realized is about 8 .mu.m at smallest from the economical and technical standpoint of view. At present, various methods for producing a toner having a reduced particle diameter are under study. However, the grinding and classification method merely provides a small particle diameter having the same particle size distribution as that of the conventional products. The particle size distribution characteristics of the toner can be hardly improved. As a result, the presence of toner particles having a smaller particle size than the other side in the distribution worsens troubles such as stain on carrier and photoreceptor and toner scattering, making it difficult to realize both high quality and high reliability at the same time.
In order to solve these problems, the process for the production of toners using various polymerization processes other than knead-grinding process is under study. For example, the process for the preparation of toners by suspension polymerization process is described in JP-A-62-73276 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-5-027476. However, the particle size distribution of the toner prepared by these processes is no better than that provided by the knead-grinding process no matter how it is controlled. In many cases, further classification is required. The toner obtained by these processes is also disadvantageous in that since the toner particles are in almost spherical form, the toner remaining on the photoreceptor or the like can be hardly removed, impairing the reliability in image quality.
Further, the process for the preparation of toner by emulsion polymerization process is described in JP-A-6-250439. However, this preparation process comprises preparing a particulate resin dispersion by an emulsion polymerization process using a surface active agent while preparing a coloring agent dispersion having a coloring agent dispersed in a solvent, mixing the two dispersions, adding a surface active agent having a polarity opposite to that of the foregoing surface active agent to the mixture so that the emulsion polymerization particles and coloring agent are agglomerated to a desired particle diameter, adding a surface active agent having the same polarity as that used in the preparation of the particulate resin to the agglomerate so that the agglomerated particles are stabilized to a desired particle diameter, and then heating the agglomerate to a temperature of not lower than the glass transition point of the binder resin so that it is fused to prepare a toner.
In accordance with the foregoing preparation process, not less than 80% of the residual surface active agent is added at the step of agglomerating the particulate resin and the particulate coloring agent and the subsequent heat-fusion step where the agglomerated particles are restabilized. Therefore, if the amount of the surface active agent to be used at the agglomeration step and the subsequent heat-fusion step is restricted to not more than a predetermined level to solve the foregoing various problems of the remaining surface active agent, some troubles occur. For example, these particles can be less fairly agglomerated, deteriorating the particle size distribution or producing unagglomerated particles. Further, these particles can be understabilized at the heat-fusion step, deteriorating its particle size distribution. Accordingly, mere reduction of the amount of the surface active agent to be used results in great problems in the production process.
Moreover, the toner particles obtained by these processes are advantageous in that they have an extremely excellent particle size distribution as compared with those obtained by polymerization processes such as conventional suspension polymerization process and can be obtained in amorphous form from the standpoint of cleaning properties. However, the toner obtained by emulsion polymerization process exhibits remarkably deteriorated moisture-absorption characteristics due to surface active agents remaining therein. As a result, the toner exhibits a deteriorated chargeability, a high environmental dependence and a deteriorated mechanical strength and hence leaves much to be desired in reliability and durability.
Further, the merely amorphous toner obtained by the foregoing process exhibits good cleaning properties but an insufficient transferability from the electrostatic image carrier that causes a remarkable drop of developability of toner.